This invention relates to a method and an apparatus for torque ripple reduction in electric motors.
Electric power steering (EPS) has been the subject of development by auto manufacturers and suppliers for over a decade because of its fuel economy and ease-of-control advantages compared with traditional hydraulic power steering (HPS). However, commercialization of EPS systems has been slow and is presently limited to small and micro-class cars because of cost and performance challenges. Among the most challenging technical issues is the annoying pulsating feel at the steering wheel and the audible noise associated with the type of high performance electric drives needed to meet the steering requirements.
The choice of motor type for an EPS is a crucial one, because it determines the characteristics of the drive and the requirements on the power switching devices, controls, and cost. Leading contenders are the permanent magnet (PM) brushless motor, the permanent magnet (PM) commutator-type and the switched reluctance (SR) motors, each of the three options has its own inherent advantages and limitations.
For the purposes of this invention, PM brushless motors are preferred over commutator-type motors. The large motor size and rotor inertia of commutator-type motors limit their applicability to very small cars with reduced steering assist requirements. Additionally, the potential for brush breakage that may result in a rotor lock necessitates the use of a clutch to disconnect the motor from the drive shaft in case of brush failure. SR drives offer an attractive, robust and low cost option, but suffer from inherent excessive torque pulsation and audible noise, unless special measures are taken to reduce such effects. For column assist applications, the motor is located within the passenger compartment and therefore must meet stringent packaging and audible noise requirements that the present SR motor technology may not satisfy. Therefore, the PM brushless motor with its superior characteristics of low inertia, high efficiency and torque density, compared to commutator motors, appears to have the potential for not only meeting the present requirements but also of future high performance EPS systems of medium and large vehicles.
Despite the relatively low levels of torque ripple and noise of EPS systems using conventional PM brushless motors, they are no match to the smoothness and quietness of HPS with decades-long history of performance refinement efforts. Consumers are reluctant in compromising such features. Therefore, a new torque ripple free (TRF) system is needed, which as the name indicates would eradicate the sources of torque ripple (under ideal conditions) and reduces the noise level considerably. The near term goal is to enhance the performance of EPS systems with the long term objective of increasing acceptability of EPS systems for broader usage.
Several performance and cost issues have stood in the way of broad-based EPS commercialization regardless of the technology used, but with varying degree of difficulty. This requires that following be addressed:
1. Steering Feel: The key to the wider use of EPS is the ability to reproduce the smoothness feel of hydraulic steering systems at affordable prices. Pulsating torque produced by motors would be felt at the steering wheel, if not reduced to very low levels.
2. Audible Noise: The EPS audible noise is mainly emanating from the motor and gearbox. The gear noise is obviously mechanical due to lash caused by manufacturing tolerances. The motor-caused noise is mainly a result of structural vibration excited by torque pulsation and radial magnetic forces in brushless motors and by the commutator/brush assembly in commutator motors.
Typically, to get torque ripple free motor from a sinusoidally excited motor, the induced voltage need to be sinusoidal without any harmonics other than the third harmonics resulting from an analysis such as Fourier analysis. Normally this is achieved by distributing the stator conductors to get a sinusoidal distribution with complementary structures on a stator of the motor.
The present invention offers advantages and alternatives over the prior art in providing a method and apparatus for torque ripple reduction in sinusoidally excited brushless permanent magnet motors. In practice, a so-called sinusoidal composition of the sinusoidally excited brushless permanent magnet motors is not an ideal or perfect sinusoidal form. Thus, based upon Fourier analysis, it is desirous to minimize or eliminate the unwanted higher order components of the sinusoidal composition.
In an exemplary embodiment of the invention, a method for determining a dimension in a motor is described. By applying Fourier analysis, a sequence of terms is obtained. Since the fifth harmonic is the most undesirable term, the minimization of the fifth harmonic term will make resultant waveform closer to sine wave. Based upon the above, a determination of an angle xcex4 is described, wherein the fifth harmonic term of the sequence of terms is minimized.
In addition, an electric motor having a rotor and a set of slot on said rotor surface having a set of magnets with a width xcex4 along the circumference of said rotor surface is described. The width xcex4 is determined by a method that includes applying Fourier analysis thereby a sequence of terms is obtained. Since the fifth harmonic is the most undesirable term, the minimization of the fifth harmonic term will make resultant waveform closer to sine wave. Based upon the above, a determination of an angle xcex4 is described, wherein the fifth harmonic term of the sequence of terms is minimized.